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Full text of "The Cambridge Natural History"

ix                           DENTITION OF TYPOTHERIA                       2 I 3
account of the genus Protypotherium. This animal was of
about the size of a Sh/rax, which indeed it resembles in several
points of structure. The teeth have the primitive number of
forty-four, and they are close set, leaving no diastema; the
molars are rootless and grow persistently; they are simple and
Rodent-like in surface pattern. The shape of the lower jaw is
like that of Hyrcux^ being rounded in outline posteriorly; there
is no projecting angle as in the Hodents, and this remark
applies to the Typotheria in general. The aspect of the Rodent
lower jaw is characteristically different from that of J3kfra® and
the forms under consideration.
Some other characters of these early forms of Typotheria can
be gathered from an inspection of other genera. In IcocJiilus
both hand and foot were five-toed, and, as in ancient Ungulates
generally, the bones of the wrist and of the ankle are serially
and not alternately arranged. Moreover, an os cent rale is
present in the carpus. Both thumb and big toe were opposable.
The skull has a remarkably Hodent-like appearance, but the
palate is not so narrowed as in these animals.
In the more recent forms of Typotheria the dentition has
become reduced. The canines are lost, and as the incisors are
reduced also, to one on each side of the upper, and two on each
side of the lower jaw, the likeness to a Hodent skull is increased.
There is also evidence of a modification from the more primitive
forms in the loss of one premolar or even more, in the alternating
bones of the carpus, in the disappearance of the centrale, and in
the loss of a toe upon the hind-foot. In these more recent
forms the fibula articulates with the astragalus instead of with
the calcaneum. Typotheria of these more recent forms may be
illustrated by the typical genus Typotlieriwrn. It has the re-
duced dental formula I-J- C -g Pm -J- M f-; the molars are simple
in pattern, and much like those of To&odon. The tipper incisors
are powerful and curved, but are surrounded by a layer of enamel,
which is not limited to the anterior face, as it practically is in
Hodents. The sacrum is composed of a large number of verte-
brae—some seven—a state of affairs which recalls the Edentata.
The shoulder blade is not Ungulate in form. It has a strong
spine, with an acromion and a well-developed metacromion, The
terminal phalanges are enlarged and hoof-like.
In the genus Pacliyrucos there are three premolars, otherwise